1. Field of the Invention
This disclosure is directed to a sample stacking method using on-line automatic solid phase extraction coupled to nonaqueous capillary electrophoresis, and an interface structure between a solid-phase preconcentration cartridge and a capillary therefor. In particular, the method is directed to a technique for significantly enhancing sensitivity in the analysis of a sample by coupling a solid-phase preconcentration cartridge to a capillary, desorbing the extracted sample with a large amount of an organic solvent, and then applying field-amplified sample stacking.
2. Description of the Related Art
In order to overcome a low detection limit of capillary electrophoresis, various methods based on electrophoresis and chromatography have been developed. The methods based on electrophoresis are methods for stacking a sample according to a difference in the mobility of the sample in discontinuous regions. The methods are advantageous in that no other special equipment is required. However, there is a disadvantage in that an additional purification process is necessary for its application to a complex sample. That is, since an additional process is required to prepare a sample, there is a problem of efficiency caused by using the methods for stacking the sample according to a difference in the mobility of the sample in discontinuous regions.
On the other hand, the methods using chromatography are methods for stacking a sample by adsorbing a large amount of the sample onto a stationary phase. The methods are advantageous in that simultaneous purification and stacking of the sample is possible. Among the methods using chromatography, solid phase extraction is most frequently used, and the basic principle of the method is as follows. When a large amount of a solution containing a sample is passed through a sorbent, the sample is adsorbed onto the sorbent. At this time, the substances in the sample solution which did not adsorb onto the sorbent are passed through, thereby allowing purification of the sample. Then, the sample is desorbed with a small volume of an elution solvent for sample stacking. If the sample is not lost during desorption, it is possible to stack the sample in the same volume ratio as the sample solution and the elution solvent. However, the volume of the solution to be injected into capillary electrophoresis is at a nanoliter level. Thus, in general, only a partial amount of the elution solvent is injected, and most of the adsorbed sample is not analyzed. There are various methods to couple solid phase extraction to capillary electrophoresis. One method includes directly connecting a solid-phase preconcentration cartridge to a capillary. This method can be applied to a typical capillary electrophoresis machine. Thus, the method is easily automatized. However, when desorption is carried out with a large amount of a solvent, the stacking efficiency is decreased resulting in no separation between the sample. Therefore, the amount of the desorption solvent is limited to a several nanoliter level, resulting in a disadvantage in that a large amount of the sample remains on the sorbent. Thereby, the residual sample influences the subsequent experiment. In short, when the sample is desorbed with an insufficient amount of the elution solvent, most of the analytes remain in the solid phase causing problems. When the sample is desorbed with a large amount of the elution solvent, there are problems of dilution and deteriorated separation.
In solid phase extraction, a polyethylene sleeve or the like is charged with a packing material and the packing material is fixed with glass fibers. The sleeve is connected between a capillary, which is cut in the middle, and adhered. Thus, these adhered substances may dissolve in a solvent resulting in decreased stability. The sleeve may be formed longer to adhere with the capillary more stably. However, in this case, one end of the connected capillary must be shortened, thereby having restrictions in installing the capillary in a commercialized capillary electrophoresis (“CE”) machine. Moreover, there is a problem in having to connect the capillary at both ends of the sleeve in addition to having a difficulty in packing the material inside the sleeve. Since one end of the capillary is shorter as mentioned above, a large amount of solvent cannot be flowed therethrough. Thus, a significant amount of analytes remain in the solid phase. This can also cause a problem of deteriorating the electrophoretic ability by having the residual analytes in the solid phase elute during the analysis.